The invention relates generally to the field of shock absorbers or energy absorption or energy dissipation devices, and particularly to fluid displacement-type shock absorbers.
Shock absorbers are normally designed to decelerate a load, i.e., a moving mass, to rest without damage. Most loads have a deceleration limit expressed in G's as a multiple of the effect of gravity. Approaching or exceeding the G limit by stopping too abruptly, risks substantial damage to the load itself. Deceleration which is too abrupt can burst hydraulic shock absorbers. Moreover, since the shock absorber transmits force to the structure on which it is mounted, the mechanical strength of the structure must also be taken into account, particularly if a load may have a positive velocity at the end of the stroke of the shock absorber, and structural or mechanical stops are used to position such load systems, wherein, the remaining energy of these systems are absorbed elastically by the restraining structure.
There are many industrial applications, for example, railroads or foundries, where very heavy loads are encountered requiring very large stopping forces. In a foundry, for example, where large metal castings are made, the sand molds into which the molten metal is poured, referred to as the "cope" and "drag", are conveyed to and from their respective stations on a "head carriage". These carriages, weighing on the order of 50,000 pounds, are generally accelerated to velocities of 5 feet per second by pneumatic cylinders which apply forces on the order of 15,000 pounds. Typical hydraulic fluid displacement-type shock absorbers used for this type of application have bore sizes ranging from 3 to 4 inches, and piston strokes or displacements of 6 to 8 inches. It is conventional in this type of shock absorber to provide some means of diminishing the orifice area used to control the rate of fluid flow out of the cylinder, under the action of the piston, into a reservoir of some kind. This can be accomplished with a plurality of axially spaced holes through the cylinder wall. As these holes are passed up by the piston head, they are covered and no longer are available as exit ports for the fluid. The size and spacing of the orifice holes used determines the deceleration characteristics that can be provided by such devices. An example in the railway industry is referred to in U.S. Pat. No. 3,301,410 to Seay.
One of the problems in industrial applications such as foundries is accommodating the wide variety of load systems encountered, whether due to variations of mass and/or velocity alone or in combination with constant or varying propelling forces. In very simple terms, a relatively stiff shock absorber is needed for a heavy mass-high intensity load system, and a relatively soft shock absorber is needed for a light mass-low intensity load system. Conventional shock absorbers are designed to handle constant mass-constant intensity load systems.
The conventional way to accommodate a variety of constant mass-constant intensity load systems is to use what has been called an "adjustable" shock absorber having some means of mechanically adjusting or presetting the relative size of the orifices in a multi-port hydraulic shock absorber, as shown, for example, in U.S. Pat. No. 4,071,122 to Schupner, owned by the assignee of this application. While it is generally understood that the most efficient way to arrest a constant mass-constant intensity load system is to provide a constant level of resistance over the entire stroke of the shock absorber, and thereby, constant deceleration, the design efficiency of conventional adjustable shock absorbers is seriously hampered by the inability to reach an optimum preadjustment for the shock absorber. Such preadjustment not only requires advance knowledge of the exact mass, and intensity of the load system which will be encountered, and the ability to pre-establish the optimum adjustment setting required without use of expensive electronic instrumentation but also that the intensity of the load system remain constant throughout the deceleration excursion. Once adjusted for a specific constant mass-constant intensity load system, the conventional shock absorber can only handle small deviations from the exact mass, and intensity of this load system. For example, it cannot efficiently stop a load system whose mass may be lower or higher than that accounted for by the adjustment setting utilized, or whose intensity tends to vary over the stroke due to increasing propelling force. Moreover, conventional adjustable shock absorbers are only provided with one mode of adjustability, that is, the size of their orifices can be adjusted but their locations cannot be. The conventional adjustable shock absorber can therefor do no more than adjust for constant mass-constant intensity load systems, for example, by rotating a sleeve to eclipse the orifices in a fixed spaced hole system as shown, for example, in U.S. Pat. No. 4,071,122. This type of sleeve structure also introduces a temperature-dependent error factor due to leakage, as some of the hydraulic fluid leaving the orifices flows between the outside of the pressure tube, containing the orifices, and the inside of the adjustment sleeve, containing the adjustment apertures, thereby bypassing the controlling apertures.